Random access resource configuration

ABSTRACT

The invention relates to a method of configuration of random access resources in the case of carrier aggregation wherein one or more uplink and downlink component carriers can be configured by the network, the method comprising the resolution of carrier ambiguity in case of downlink and uplink asymmetric component carrier (CC) configuration by allowing the network to determine on which downlink component carrier the UE camps on. The invention further relates to a method of random access and a network entity and a user equipment for implementing the methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. national stage application ofInternational Application No. PCT/KR2009/006848, filed on Nov. 20, 2009,which claims priority to U.S. Provisional Application Ser. Nos.61/159,060, filed on Mar. 10, 2009 and 61/149,335, filed on Feb. 2,2009, the contents of which are incorporated by reference herein intheir entirety.

FIELD OF THE INVENTION

The present invention relates to a random access channel (RACH)procedure in a cellular communications network, and in particular to amethod and apparatus for configuring random access resources. While itis described below in the context of a long term evolution (LTE) andLTE-A (long term evolution advanced) type cellular network forillustrative purposes and since it happens to be well suited to thatcontext, those skilled in the art will recognise that the inventiondisclosed herein can also be applied to various other types of cellularnetworks.

DISCUSSION OF THE RELATED ART

A universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in Wideband CodeDivision Multiple Access (WCDMA) based on a European standard known asGlobal System for Mobile Communications (GSM), and general packet radioservices (GPRS). The LTE of UMTS is under discussion by the 3rdgeneration partnership project (3GPP) that standardised UMTS.

3GPP LTE is a technology for enabling high-speed packet communications.Many schemes have been proposed for the LTE objective including thosethat aim to reduce user and provider costs, improve service quality, andexpand and improve coverage and system capacity. 3G LTE requires reducedcost per bit, increased service availability, flexible use of afrequency band, a simple structure, an open interface, and adequatepower consumption of a terminal as an upper-level requirement.

FIG. 1 is a block diagram illustrating a network structure of an evolveduniversal terrestrial radio access system (E-UTRA). The E-UTRA may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voiceand packet data.

As illustrated in FIG. 1, the E-UTRA network includes an evolved UMTSterrestrial radio access network (E-UTRAN) and an evolved packet core(EPC) and one or more user equipments (UEs) 101. The E-UTRAN may includeone or more evolved NodeBs (eNodeB, or eNB) 103, and a plurality of UEs101 may be located in one cell. One or more E-UTRAN mobility managemententity (MME)/system architecture evolution (SAE) gateways 105 may bepositioned at the end of the network and connected to an externalnetwork.

As used herein, “downlink” refers to communication from an eNodeB 103 toa UE 101, and “uplink” refers to communication from the UE 101 to aneNodeB 103. UE 101 refers to communication equipment carried by a userand may be also be referred to as a mobile station (MS), a user terminal(UT), a subscriber station (SS) or a wireless device.

An eNodeB 103 provides end points of a user plane and a control plane tothe UE 101. MME/SAE gateway 105 provides an end point of a session andmobility management function for UE 101. The eNodeB 103 and the MME/SAEgateway 105 may be connected via an S1 interface.

The eNodeB 103 is generally a fixed station that communicates with a UE101, and may also be referred to as a base station (BS), a networkentity or an access point. One eNodeB 103 may be deployed per cell. Aninterface for transmitting user traffic or control traffic may be usedbetween eNodeBs 103.

The MME provides various functions including distribution of pagingmessages to eNodeBs 103, security control, idle state mobility control,SAE bearer control, and ciphering and integrity protection of non-accessstratum (NAS) signalling. The SAE gateway host provides assortedfunctions including termination of U-plane packets for paging reasons,and switching of the U-plane to support UE mobility. For clarity,MME/SAE gateway 105 will be referred to herein simply as a “gateway,”but it is understood that this entity includes both an MME and an SAEgateway.

A plurality of nodes may be connected between the eNodeB 103 and thegateway 105 via the S1 interface. The eNodeBs 103 may be connected toeach other via an X2 interface and neighbouring eNodeBs may have ameshed network structure that has the X2 interface.

FIG. 2( a) is a block diagram depicting architecture of a typicalE-UTRAN and a typical EPC. As illustrated, eNodeB 103 may performfunctions of selection for gateway 105, routing toward the gatewayduring a radio resource control (RRC) activation, scheduling andtransmitting of paging messages, scheduling and transmitting ofbroadcast channel (BCCH) information, dynamic allocation of resources toUEs 101 in both uplink and downlink, configuration and provisioning ofeNodeB measurements, radio bearer control, radio admission control(RAC), and connection mobility control in LTE ACTIVE state. In the EPC,and as noted above, gateway 105 may perform functions of pagingorigination, LTE-IDLE state management, ciphering of the user plane,system architecture evolution (SAE) bearer control, and ciphering andintegrity protection of non-access stratum (NAS) signalling.

FIGS. 2( b) and 2(c) are block diagrams depicting the user-planeprotocol and the control-plane protocol stack for the E-UMTS. Asillustrated, the protocol layers may be divided into a first layer (L1),a second layer (L2) and a third layer (L3) based upon the three lowerlayers of an open system interconnection (OSI) standard model that iswell-known in the art of communication systems.

The physical layer, the first layer (L1), provides an informationtransmission service to an upper layer by using a physical channel. Thephysical layer is connected with a medium access control (MAC) layerlocated at a higher level through a transport channel, and data betweenthe MAC layer and the physical layer is transferred via the transportchannel. Between different physical layers, namely, between physicallayers of a transmission side and a reception side, data is transferredvia the physical channel.

The MAC layer of Layer 2 (L2) provides services to a radio link control(RLC) layer (which is a higher layer) via a logical channel. The RLClayer of Layer 2 (L2) supports the transmission of data withreliability. It should be noted that the RLC layer illustrated in FIGS.2( b) and 2(c) is depicted because if the RLC functions are implementedin and performed by the MAC layer, the RLC layer itself is not required.The packet data convergence protocol (PDCP) layer of Layer 2 (L2)performs a header compression function that reduces unnecessary controlinformation such that data being transmitted by employing Internetprotocol (IP) packets, such as IPv4 or IPv6, can be efficiently sentover a radio (wireless) interface that has a relatively small bandwidth.

A radio resource control (RRC) layer located at the lowest portion ofthe third layer (L3) is only defined in the control plane and controlslogical channels, transport channels and the physical channels inrelation to the configuration, reconfiguration, and release of the radiobearers (RBs). Here, the RB signifies a service provided by the secondlayer (L2) for data transmission between the terminal and the E-UTRAN.

As illustrated in FIG. 2( b), the RLC and MAC layers (terminated in aneNodeB 103 on the network side) may perform functions such asscheduling, automatic repeat request (ARQ), and hybrid automatic repeatrequest (HARQ). The PDCP layer (terminated in eNodeB 103 on the networkside) may perform the user plane functions such as header compression,integrity protection, and ciphering.

As illustrated in FIG. 2( c), the RLC and MAC layers (terminated in aneNodeB 103 on the network side) perform the same functions as for thecontrol plane. As illustrated, the RRC layer (terminated in an eNodeB103 on the network side) may perform functions such as broadcasting,paging, RRC connection management, RB control, mobility functions, andUE measurement reporting and controlling. The NAS control protocol(terminated in the MME of gateway 105 on the network side) may performfunctions such as an SAE bearer management, authentication, LTE_IDLEmobility handling, paging origination in LTE_IDLE, and security controlfor the signalling between the gateway and UE 101.

The NAS control protocol may use three different states; first, aLTE_DETACHED state if there is no RRC entity; second, a LTE_IDLE stateif there is no RRC connection while storing minimal UE information; andthird, an LTE ACTIVE state if the RRC connection is established. Also,the RRC state may be divided into two different states such as aRRC_IDLE and a RRC_CONNECTED.

In RRC_IDLE state, the UE 101 may receive broadcasts of systeminformation and paging information while the UE 101 specifies adiscontinuous reception (DRX) configured by NAS, and the UE has beenallocated an identification (ID) which uniquely identifies the UE in atracking area. Also, in RRC-IDLE state, no RRC context is stored in theeNodeB 103.

In RRC_CONNECTED state, the UE 101 has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the network (eNodeB) becomes possible. Also, the UE 101 canreport channel quality information and feedback information to theeNodeB 103.

In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 101belongs. Therefore, the network can transmit and/or receive data to/fromthe UE 101, the network can control mobility (handover) of the UE 101,and the network can perform cell measurements for a neighbouring cell.

In RRC_IDLE mode, the UE 101 specifies the paging discontinuousreception (DRX) cycle. Specifically, the UE 101 monitors a paging signalat a specific paging occasion of every UE specific paging DRX cycle.

The procedure where a UE sends a first message to a network is commonlyreferred to as initial access. In most systems the initial access isinitiated by a UE transmitting a connection request message includingthe reason of the request, and receiving an answer from the networkindicating the allocation of radio resources for the requested reason.

In 3GPP TS 25.331 there are several reasons, referred to asestablishment causes, for sending a connection request message.Establishment causes include: originatingconversational/streaming/interactive/background/subscribed traffic call,terminating conversational/streaming/interactive/background call,emergency call, inter radio access technology (RAT) cell re-selection,inter-RAT cell change order, registration, detach, originating high/lowpriority signalling, call re-establishment and terminating high/lowpriority signalling.

An “originating call” establishment indicates that the UE 101 wishes tosetup a connection, for instance a speech connection. A “terminatingcall” establishment indicates that the UE 101 answers to paging. A“registration” establishment indicates that the user wants to registeronly to the network.

To initiate access to the network a random access procedure is used. Thephysical random access transmission is under the control of higher layerprotocol which performs some important functions related to priority andload control. These procedures differ in detail but GSM, UMTS and LTEradio systems have some similarities between them.

In the random access procedure the UE 101 randomly selects an accessresource and transmits a RACH preamble to the network. A preamble is ashort signal that is sent before the transmission of the RACH connectionrequest message. The UE 101 can repeatedly transmit the preamble byincreasing the transmission power each time the preamble is sent untilthe network indicates the detection of the preamble. The message partcan then be sent at the level of power equal of the last preambletransmission power plus an offset signalled by the network.

A random access channel (RACH) is a common physical channel dedicated tothe random access procedure. Uplink transmissions are generallyinitiated through a RACH. A UE sending data on a RACH has not yet beenidentified by the target eNB. RACH is typically an uplink common channelused for transmitting control information and user data. It is appliedin random access, and used for low-rate data transmissions from thehigher layer. Such a channel is said to be contention-based since manyusers can attempt to access the same base station simultaneously,leading to collisions. A RACH channel can be used for several purposes.For example the RACH can be used to access the network, to requestresources, to carry control information, to adjust the time offset ofthe uplink in order to obtain uplink synchronisation, to adjust thetransmitted power, etc.

A random access procedure can be launched by the UE or the eNodeB. Itmay, for instance, be triggered by the following events:

a UE switches from power-off to power-on and needs to be registered tothe network.

a UE is not time-synchronized with a eNodeB and starts transmitting data(for instance the user calls).

a eNodeB starts transmitting data to the UE but they are notsynchronized (for instance the user receives a call).

a eNodeB measures a delay of the received signal from the UE (forinstance the user is moving and has lost synchronization).

a UE is moving from one cell to another and needs to betime-synchronized with a different target eNodeB than the serving eNodeBit is registered to (handover).

In LTE, the basic unit of time is a slot (generally of a duration of 0.5ms). Two slots make up a subframe and ten subframes constitute a radioframe. A random access channel typically occupies 6 resource blocks in asubframe or set of consecutive subframes reserved for random accesspreamble transmissions. A RACH period can be configured to be, forexample, 1 ms, 2 ms, 5 ms and 10 ms. FIG. 3 shows one possible mappingof the RACH within a resource grid.

FIG. 4 illustrates an example of the sequences of messages and responsesexchanged between a user equipment UE 101 and a base station eNB 103 ina typical RACH procedure.

Firstly the UE 101 retrieves information transmitted periodically fromeNB 103 on a downlink broadcast channel (BCH). The received informationincludes the available preamble signatures in the cell, the location andperiod of RACH time slots; From the received information the UE 101selects a preamble signature, a RACH time slot and a frequency band.

The preamble signature is chosen by the UE 101 from among a set ofpreamble signatures known by the eNB 103. The UE 101 generates a singlerandom access burst containing the chosen preamble signature andtransmits it to the eNB 103 over the selected time slot at the selectedfrequency in message 1.

The random access burst consists of a cyclic prefix, a preamble, and aguard time during which nothing is transmitted as illustrated in FIG. 5.CP denotes cyclic prefix, GT denotes guard time, RTD denotes round tripdelay and TTI denotes transmission time interval.

The preamble is sent before a RACH connection request and indicates thatthe UE is about to transmit data. The random access burst is transmittedduring one subframe. While the UE is not synchronized in the timedomain, its random access burst may overlap with the next subframe andgenerate interference. A guard time may thus be added to combatinterference. The guard time (GT) should be at least equal to theround-trip delay at the cell edge.

During the random access procedure, several users share the samechannel. They are distinguishable by virtue of orthogonal sequences.These sequences are seen as the UE preamble signatures that can betransmitted simultaneously. A collision occurs whenever several userschoose the same signature and send it within the same time and frequencyresources.

Preamble signatures should portray good autocorrelation properties inorder for the eNodeB 103 to obtain an accurate timing estimation for asingle preamble; and good cross correlation properties in order for theeNodeB 103 to obtain an accurate timing estimation for differentpreambles transmitted simultaneously by different UEs.

The Zadoff-Chu Zero Correlation Zone (ZC-ZCZ) sequences are used tofulfil these requirements. Each cell possesses a set of 64 signaturesobtained from ZC-ZCZ sequences. The length of one sequence is N=839samples. A ZC-ZCZ sequence is defined by two integers: u is the rootindex and v is the cyclic shift index.

In the time domain, the v-th cyclic shift is extracted from the u throot with:x _(u,v)(n)=x _(u)(n+v·N _(CS)) n=0 . . . N−1where N_(CS) is the cyclic shift length.

The u-th root sequence in the frequency domain is given by:

${x_{u}(n)} = {\mathbb{e}}^{{\mathbb{i}}\;{\pi \cdot u \cdot \frac{n{({n + 1})}}{N}}}$

The ZC-ZCZ sequences are used because they can generate a large numberof sequences and they offer interesting correlation properties: theautocorrelation function shows no side peaks. The cross correlationbetween two sequences obtained from different roots is √{square rootover (N)}. Thus ZC sequences have zero-cross-correlation zones.

The eNB 103 monitors the current RACH slot in an attempt to detectpreambles transmitted from UEs in the corresponding cell.

On reception of a signal the eNB 103 correlates the received signal inthe RACH sub-frame with all possible signatures. Detection of thepreamble can be either performed in the time domain or in the frequencydomain. A detection variable is computed for each signature. If thedetection variable exceeds a certain threshold, the preamble isconsidered detected.

The eNB 103 sends a random access response to acknowledge thesuccessfully detected preambles in message 2. This message is sent on adedicated downlink channel and uses the detected signature. It containsa timing advance command, a power-control command. If the procedure iscontention-free then the UE and the eNodeB are thereby aligned in thetime domain.

If the UE 101 receives a response from the eNB 103 the UE 101 decodesthe response and adapts its transmission timing, and its transmissionpower if the response contains power control information. The UE 101then sends a resource request message—message 3—on a dedicated uplinkchannel. In this message, the UE requests bandwidth and time resourcesto transmit data and it also indicates a UE-specific identifier. If theUE requests resources, the UE 101 uses a specific ID in the message toresolve contentions. Then the UE monitors a specified downlink channelfor response from the eNB. In the case of a positive resource grant, thesubsequent transmissions are carried out as normal.

The eNB attempts to resolve any contentions. If the eNB 103 receives aresource request with a UE-specific signature the eNB 103 checks howmany UEs were detected with the same signature and resolves any possiblecontentions. If the preamble sent by UE 101 was in collision with apreamble from another UE, the eNB 103 sends a contention resolutionmessage—message 4—to give the command to UE 101 to re-start the RACHprocedure. If on the other hand the UE 101 was not in collision, the eNBsends a resource assignment message—message 5. In this case thesubsequent transmissions are carried out as usual. The eNB 103identifies the UE 101 and assigns resources according to the schedulingrules applied.

In the random access response, message 2, the UE may receive an ACKsignal from the eNB to indicate that a message can be sent, a NACKsignal indicating that the preamble was detected but a message cannot tobe sent, or no response indicating that the preamble was not detected.

In the case where UE 101 receives no response indicating that a preamblehas not been detected at the first attempt the UE 101 waits for the nextRACH slot to send another preamble. The preamble signal-to-noise ratio(SNR) is relatively low compared to data SNR owing to the length of thezero-correlation sequences. Given that the random access channel doesnot generate much interference, the UE can afford to increase thetransmission power by a few decibels (dB) at the second attempt toprevent consecutive failures (power ramping method). A too long delay isnot desirable, especially in the case of handovers. The UE 101repeatedly transmits the preamble by increasing the transmission powerevery time the preamble is sent until the network indicates thedetection of the preamble. The procedure is exited after a certainnumber of failures. If a preamble is successfully transmitted themessage part is generally sent at the level of power equal to the lastpreamble transmission power plus an offset signaled by the network.

In LTE-A (Long Term Evolution—Advanced) the employment of carrieraggregation where two or more component carriers has been considered inorder to provide increased transmission bandwidth and to supportspectrum aggregation. The LTE-A system supports transmission bandwidthsof up to 100 MHz. Carrier aggregation between uplink (UL) and downlink(DL) bandwidth can be either symmetric or asymmetric. In the case ofsymmetric carrier aggregation (e.g. 2UL component carriers and 2DLcomponent carriers) if 2UL component carriers have different PRACHconfigurations, the processing complexity of an eNB can increase by afactor of 2. In the case of asymmetric carrier aggregation as,illustrated in FIG. 6, (e.g. 2 CCs in DL and 1 CC in UL) there is anambiguity issue to solve, since an eNB has no knowledge as to whichdownlink component carrier a UE is camped on in order to send a randomaccess response to the UE. As a result it may send a response on bothdownlink component carriers. This ambiguity can impact an eNB'sbehaviour in the rest of the random access procedure and result inresource waste.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of configuration of random access resources in the case ofcarrier aggregation wherein one or more uplink and downlink componentcarriers can be configured by the network, the method comprising:allocating, by each of a plurality of downlink component carriers, arespective PRACH configuration for at least one corresponding uplinkcomponent carrier; allocating, by each downlink component carrier arespective PRACH frequency position, for the at least one correspondinguplink component carrier; assigning, by each downlink component carrier,a respective resource access pattern, for the at least one correspondinguplink component carrier wherein the resource access pattern defines atime pattern of available PRACH resources in a radio frame; andtransmitting the allocated PRACH configuration, the allocated PRACHfrequency position, and the assigned resource access pattern on thecorresponding downlink component carrier. In embodiments of theinvention, the same PRACH configuration and/or the same PRACH frequencyposition may be allocated for the uplink component carriers by theplurality of downlink component carriers.

The solution proposed by the invention is applicable to either symmetricor asymmetric carrier aggregation, i.e. it constitutes an agnosticsolution. Since the same PRACH configuration can be allocated to all ULcomponent carriers the processing complexity is minimized and can remainat the same level of complexity as for LTE Rel8. The PRACHvalidity/access pattern allocated to each UL component carrier helps tosolve the ambiguity issue that exists in an asymmetric case byexploiting random access opportunities.

According to a second aspect of the present invention there is provideda method of random access transmission, the method comprising receivinga PRACH configuration on a downlink component carrier; receiving a PRACHfrequency position to the downlink component carrier; receiving aresource access pattern on the downlink component carrier wherein theresource access pattern defines a time pattern of available PRACHresources in a radio frame; selecting an available PRACH resourceaccording to the resource access pattern; and transmitting a RACHpreamble on the selected PRACH resource.

According to a third aspect of the invention there is provided a userequipment comprising: a transceiver for receiving a PRACH configurationon a downlink component carrier; receiving a PRACH frequency position onthe downlink component carrier; receiving a resource access pattern onthe downlink component carrier wherein the resource access patterndefines a time pattern of available PRACH resources in a radio frame; aselector for selecting an available PRACH resource according to theresource access pattern; wherein the transceiver is operable to transmitthe RACH preamble on the selected PRACH resource.

According to a fourth aspect of the invention there is provided anetwork entity for configuring random access resources in the case ofcarrier aggregation wherein one or more uplink and downlink componentcarriers can be configured by the network, the network entitycomprising: a PRACH configuration allocator for allocating, by each of aplurality of downlink carriers, a PRACH configuration for at least onecorresponding uplink component carrier; a PRACH frequency positionallocator for allocating, by each of a plurality of downlink componentcarriers, a PRACH frequency position for the corresponding uplinkcomponent carrier; a resource access pattern assignor for assigning, byeach downlink component carrier, a respective resource access patternfor each corresponding uplink component carrier wherein the resourceaccess pattern defines a time pattern of available uplink PRACHresources in a radio frame; and a transceiver for transmitting theallocated PRACH configuration, the allocated PRACH frequency position,and the assigned resource access pattern on the corresponding downlinkcomponent carrier.

The methods according to the invention may be computer implemented. Themethods may be implemented in software on a programmable apparatus. Theymay also be implemented solely in hardware or in software, or in acombination thereof.

Since the present invention can be implemented in software, the presentinvention can be embodied as computer readable code for provision to aprogrammable apparatus on any suitable carrier medium. A tangiblecarrier medium may comprise a storage medium such as a floppy disk, aCD-ROM, a hard disk drive, a magnetic tape device or a solid statememory device and the like. A transient carrier medium may include asignal such as an electrical signal, an electronic signal, an opticalsignal, an acoustic signal, a magnetic signal or an electromagneticsignal, e.g. a microwave or RF signal.

BRIEF DESCRIPTION THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the following drawings in which:—

FIG. 1 is a block diagram illustrating network structure of an E-UTRAsystem.

FIGS. 2( a), 2(b) and 2(c) are block diagrams depicting logicarchitecture of typical network entities of the LTE system (FIG. 2( a)),a user-plane (U-plane) protocol stack (FIG. 2( b)) and a control-plane(C-plane) protocol stack (FIG. 2( c)).

FIG. 3 graphically illustrates an example of the location of RACH slotsin a 2.5 MHz bandwidth

FIG. 4 is a diagram illustrating a typical RACH procedure

FIG. 5 schematically illustrates a RACH preamble structure in E-UTRA

FIG. 6 is a schematic diagram of an example of asymmetric carrieraggregation

FIG. 7 is a graphical illustration of an example of an uplink PRACHconfiguration and a time validity pattern allocated by a first downlinkcomponent carrier and a second downlink component carrier according toan embodiment of the invention

FIG. 8A is an example of an uplink component carrier configuration withcommon frequency position being signaled by a first downlink componentcarrier and a second downlink component carrier according to anembodiment of the invention

FIG. 8B is an example of a uplink component carrier configuration withdifferent frequency positions being signaled by a first downlinkcomponent carrier and a second downlink component carrier according toanother embodiment of the invention

FIG. 9 is a flow chart of steps of a method of allocating random accessresources according to an embodiment of the invention

FIG. 10 is a flow chart of steps of a method of random access preambletransmission according to an embodiment of the invention

DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

Embodiments of the present invention are directed to a RACH initialaccess procedure between a UE and an eNodeB.

The transmission of a PRACH, is restricted to certain time and frequencyresources. These resources are enumerated in increasing order of thesubframe number within the radio frame and the physical resource blocksin the frequency domain such that index 0 corresponds to the lowestnumbered physical resource block and subframe within the radio frame.PRACH resources within the radio frame are indicated by a PRACH ResourceIndex, where the indexing is shown in Table 1.

The parameter PRACH-Configuration-Index is given by higher layers,indicating the available PRACH resources per subframe with PRACHopportunities of 1, 2, 5, 10, and 20 ms. The parameterPRACH-FrequencyOffset given by higher layers indicates PRACH resourcesavailable in the frequency domain.

TABLE 1 frame structure type 1 random access configuration for preambleformat 0-3 [TS.36.211] PRACH System Configuration Preamble frameSubframe Index Format number number 0 0 Even 1 1 0 Even 4 2 0 Even 7 3 0Any 1 4 0 Any 4 5 0 Any 7 6 0 Any 1, 6 7 0 Any 2, 7 8 0 Any 3, 8 9 0 Any1, 4, 7 10 0 Any 2, 5, 8 11 0 Any 3, 6, 9 12 0 Any 0, 2, 4, 6, 8 13 0Any 1, 3, 5, 7, 9 14 0 Any 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 15 0 Even 9 16 1Even 1 17 1 Even 4 18 1 Even 7 19 1 Any 1 20 1 Any 4 21 1 Any 7 22 1 Any1, 6 23 1 Any 2, 7 24 1 Any 3, 8 25 1 Any 1, 4, 7 26 1 Any 2, 5, 8 27 1Any 3, 6, 9 28 1 Any 0, 2, 4, 6, 8 29 1 Any 1, 3, 5, 7, 9 30 N/A N/A N/A31 1 Even 9 32 2 Even 1 33 2 Even 4 34 2 Even 7 35 2 Any 1 36 2 Any 4 372 Any 7 38 2 Any 1, 6 39 2 Any 2, 7 40 2 Any 3, 8 41 2 Any 1, 4, 7 42 2Any 2, 5, 8 43 2 Any 3, 6, 9 44 2 Any 0, 2, 4, 6, 8 45 2 Any 1, 3, 5, 7,9 46 N/A N/A N/A 47 2 Even 9 48 3 Even 1 49 3 Even 4 50 3 Even 7 51 3Any 1 52 3 Any 4 53 3 Any 7 54 3 Any 1, 6 55 3 Any 2, 7 56 3 Any 3, 8 573 Any 1, 4, 7 58 3 Any 2, 5, 8 59 3 Any 3, 6, 9 60 N/A N/A N/A 61 N/AN/A N/A 62 N/A N/A N/A 63 3 Even 9

In a first embodiment of the invention the proposed solution introducesa transmission of a time/frequency pattern by each downlink componentcarrier CC_(i) of n component carriers and avoids uplink (UL) overloadby each downlink component carrier transmitting the same PRACHconfiguration index. For example as illustrated in FIG. 7 odd numbereduplink PRACH opportunities in the time domain are assigned by a firstdownlink component carrier CC#1 and even numbered uplink PRACHopportunities are assigned by a second downlink component carrier CC#2.Moreover in other embodiments any specific uplink PRACH opportunitiesmay be allocated by a specific downlink component carrier or all uplinkPRACH opportunities may be allocated by one downlink component carrierCC_(i).

FIG. 9 illustrates a random access procedure in LTE-A between a userequipment UE 101 and a base station eNodeB 103 according to at least oneembodiment of the present invention.

In step S101 a PRACH configuration index is assigned by each downlinkcomponent carrier CC1 to CC_(n) of n downlink carriers CC_(i). In thefirst embodiment of the invention the same PRACH configuration isassigned by each downlink component carrier CC_(i).

In step S102 a PRACH frequency position is assigned by each downlinkcomponent carrier CC_(i). In the first embodiment of the invention thesame PRACH frequency is allocated by each downlink carrier for exampleas illustrated in FIG. 8A where a common frequency position but adifferent timing pattern is allocated by CC1 and CC2

In alternative embodiments of the invention different PRACH frequencypositions may be allocated by each downlink component carrier CC_(i) asillustrated in FIG. 8B.

In step S103 a resource access pattern is assigned by each downlinkcomponent carrier CC_(i) wherein the resource access pattern defines atime pattern of uplink available PRACH resources within a radio frame.The resources access pattern can define at least one time sub-frameoccurrence of an available PRACH resource corresponding to the allocatedPRACH configuration or define even or odd sub frame occurrences ofavailable PRACH resources corresponding to the PRACH configuration.

Depending on the possible cases of time/frequency pattern assignmentsignaled by each of the downlink component carriers CC_(i) a validitypattern code could be used to specify to indicate the assigned uplinktime/frequency pattern. The validity pattern can be independent of thePRACH configuration that is used. For example 2 bits could indicate theconfiguration depicted in FIG. 7 as presented in table 2:

TABLE 2 Example of a validity pattern independent of PRACHconfiguration. Validity pattern PRACH occasion 00 Each PRACH opportunity01 Every even PRACH opportunity 10 Every odd PRACH opportunity 11Reserved

Alternatively, the interpretation of the validity pattern could dependon the PRACH configuration. For example, for PRACH configuration index10 (PRACH in subframes 2, 5 and 8, see Table 1), a possibleconfiguration is presented in Table 3:

TABLE 3 Example of validity pattern dependant on PRACH configuration.Validity pattern PRACH occasion 00 Each PRACH opportunity 01 Eachsubframe 2 10 Each subframe 5 11 Each subframe 8

In alternative embodiments of the invention a combination and/oraggregation of the examples given in Tables 2 and 3 can be envisaged inorder to cover all possible PRACH configurations as shown in Table 4

TABLE 4 Example of validity pattern dependant of PRACH configuration.Validity Configuration pattern Validity pattern 0 0000 All 1 0001 Eachsubframe 0 2 0010 Each subframe 1 3 0011 Each subframe 2 4 0100 Eachsubframe 3 5 0101 Each subframe 4 6 0110 Each subframe 5 7 0111 Eachsubframe 6 8 1000 Each subframe 7 9 1001 Each subframe 8 10 1010 Eachsubframe 9 11 1011 Every even PRACH opportunity 12 1100 Every odd PRACHopportunity 13 1101 Reserved 14 1110 Reserved 15 1111 Reserved

A way of allocating the time/frequency validity pattern may beimplemented in embodiments of the invention as follows:

Each downlink component carrier CC_(i) is assigned and the potentialPRACH occasions restrictions due to the signaled validity pattern areconsidered for the determination of the next PRACH occasion.

To allow this, a different validity pattern is signaled by the networkwithin broadcasting information elements of each downlink CC_(i)

The validity pattern signaled is mapped to the corresponding PRACHindex.

The PRACH resources are ordered according to their occurrence in timeand frequency.

The PRACH occasions can be ordered in a way that they are addressable bya validity pattern that indicates at which occasions a PRACH can beapplied, i.e. every 1st, every 2nd and so on.

In step S104 the PRACH configuration, PRACH frequency positionallocation and resource access pattern are transmitted by the enodeB onthe corresponding downlink component carrier.

With reference to FIG. 10 in step S201 a UE receives on a downlinkcomponent carrier the PRACH configuration, the PRACH frequency positionand the resource access pattern. In step S202 the UE selects anavailable PRACH resource according to the received resource pattern. Instep S203 the UE transmits a preamble to the eNode on the selected PRACHresource.

Thus in embodiments of the invention the proposed solution introduces atransmission of the time/frequency pattern by each downlink CC_(i)avoiding the UL overload since the same PRACH configuration index may beused by each downlink component carrier CC_(i). By this, there is thepossibility to assign either all even or all odd uplink PRACHopportunities in the time domain by one downlink component carrierCC_(i). Moreover any specific PRACH opportunities or all opportunitiesmay be allocated by one downlink component carrier CC_(i) Which patternto use (validity/access pattern) is broadcasted in the PBCH of eachcomponent carrier by the eNB. The same PRACH configuration can bebroadcast by each component carrier CC_(i) and that the accessopportunity is provided by the validity/access pattern that should bedifferent for each component carrier CC_(i).

Many further modifications and variations will suggest themselves tothose versed in the art upon making reference to the foregoingillustrative embodiments, which are given by way of example only andwhich are not intended to limit the scope of the invention, that beingdetermined solely by the appended claims.

The invention claimed is:
 1. A method of random access resources at anetwork for carrier aggregation, wherein one or more uplink and downlinkcomponent carriers can be configured by the network, the methodcomprising: allocating, by each of a plurality of downlink componentcarriers, a Physical Random Access Channel configuration for at leastone corresponding uplink component carrier, wherein the Physical RandomAccess Channel configuration indicates a preamble format and at leastone available subframe number within a radio frame; allocating, by eachdownlink component carrier, a Physical Random Access Channel frequencyposition for the at least one corresponding uplink component carrier;assigning, by each downlink component carrier, a resource access patternfor each corresponding uplink component carrier, wherein the resourceaccess pattern defines a time pattern of the at least one availablesubframe number indicated by the Physical Random Access Channelconfiguration; and transmitting the allocated Physical Random AccessChannel configuration, the allocated Physical Random Access Channelfrequency position, and the assigned resource access pattern on thecorresponding downlink component carrier to a user equipment, wherein asame Physical Random Access Channel configuration is allocated to betransmitted by each downlink component carrier, wherein the resourceaccess pattern further defines at least one time sub-frame occurrence ofthe at least one available subframe number indicated by the PhysicalRandom Access Channel configuration, wherein a different resource accesspattern is assigned to be transmitted by each downlink componentcarrier, and wherein a different Physical Random Access Channelfrequency position is allocated to be transmitted by each downlinkcomponent carrier to solve ambiguities resulting from a use of each ofthe plurality of downlink component carriers.
 2. The method according toclaim 1, wherein: the resource access pattern further defines even orodd time sub-frame occurrences of the at least one available subframenumber indicated by the Physical Random Access Channel configuration; ifthe resource access pattern defines even occurrences, a Random AccessChannel preamble is transmitted on an even time sub-frame occurrence ofthe at least one available subframe number indicated by the PhysicalRandom Access Channel configuration; and if the resource access patterndefines odd occurrences, the Random Access Channel preamble istransmitted on an odd time sub-frame occurrence of the at least oneavailable subframe number indicated by the Physical Random AccessChannel configuration.
 3. A method of random access transmission at auser equipment, the method comprising: receiving a Physical RandomAccess Channel configuration on one of a plurality of downlink componentcarriers, wherein the Physical Random Access Channel configurationindicates a preamble format and at least one available subframe number;receiving a Physical Random Access Channel frequency position on each ofthe downlink component carriers; receiving a resource access pattern onthe one of the plurality of downlink component carriers, wherein theresource access pattern defines a time pattern of the at least oneavailable subframe number indicated by the Physical Random AccessChannel configuration; selecting an available Physical Random AccessChannel resource according to the resource access pattern; andtransmitting a Random Access Channel preamble on the selected PhysicalRandom Access Channel resource, wherein the resource access patternfurther defines at least one time sub-frame occurrence of the at leastone available subframe number indicated by the Physical Random AccessChannel configuration, and wherein a different Physical Random AccessChannel frequency position is allocated to be transmitted by eachdownlink component carrier to solve ambiguities resulting from a use ofeach of the plurality of downlink component carriers.
 4. The methodaccording to claim 3, wherein the resource access pattern defines evenor odd time sub-frame occurrences of the at least one available subframenumber indicated by the Physical Random Access Channel configuration; ifthe resource access pattern defines even occurrences, a Random AccessChannel preamble is transmitted on an even time sub-frame occurrence ofthe at least one available subframe number indicated by the PhysicalRandom Access Channel configuration; and if the resource access patterndefines odd occurrences, a Random Access Channel preamble is transmittedon an odd time sub-frame occurrence of the at least one availablesubframe number indicated by the Physical Random Access Channelconfiguration.
 5. A user equipment, comprising: a transceiver for:receiving a Physical Random Access Channel configuration on one of aplurality of downlink component carriers, wherein the Physical RandomAccess Channel configuration indicates a preamble format and at leastone available subframe number within a radio frame; receiving a PhysicalRandom Access Channel frequency position on the one of the plurality ofdownlink component carriers; and receiving a resource access pattern onthe downlink component carrier, wherein the resource access patterndefines a time pattern of the at least one available subframe numberindicated by the Physical Random Access Channel configuration; and aselector for selecting an available Physical Random Access Channelresource according to the resource access pattern, wherein thetransceiver is operable to transmit a random Access Channel preamble onthe selected Physical Random Access Channel resource, wherein theresource access pattern further defines at least one time sub-frameoccurrence of the at least one available subframe number indicated bythe Physical Random Access Channel configuration, and wherein adifferent Physical Random Access Channel frequency position is allocatedto be transmitted by each downlink component carrier to solveambiguities resulting from a use of each of the plurality of downlinkcomponent carriers.
 6. A network entity for configuring random accessresources for carrier aggregation, wherein one or more uplink anddownlink component carriers can be configured by the network, thenetwork entity comprising: a Physical Random Access Channelconfiguration allocator for allocating, by each of a plurality ofdownlink carriers, a Physical Random Access Channel configuration for atleast one corresponding uplink component carrier, wherein the PhysicalRandom Access Channel configuration indicates a preamble format and atleast one available subframe number within a radio frame; a PhysicalRandom Access Channel frequency position allocator for allocating, byeach of a plurality of downlink component carriers, a Physical RandomAccess Channel frequency position for the corresponding uplink componentcarrier; a resource access pattern assignor for assigning, by eachdownlink component carrier, a resource access pattern for eachcorresponding uplink component carrier wherein the resource accesspattern defines a time pattern of the at least one available subframenumber indicated by the Physical Random Access Channel configuration;and a transceiver for transmitting the allocated Physical Random AccessChannel configuration, the allocated Physical Random Access Channelfrequency position, and the assigned resource access pattern on thecorresponding downlink component carrier to a user equipment, wherein asame Physical Random Access Channel configuration is allocated to betransmitted by each downlink component carrier, wherein resource accesspattern r defines at least one time sub-frame occurrence of the at leastone available subframe number indicated by the Physical Random AccessChannel configuration, wherein a different resource access pattern isassigned to be transmitted by each downlink component carrier, andwherein a different Physical Random Access Channel frequency position isallocated to be transmitted by each downlink component carrier to solveambiguities resulting from a use of each of the plurality of downlinkcomponent carriers.